The invention relates to a method for regulating a compressed-air supply system of a motor vehicle, with the system comprising a drive engine which is the drive engine of the motor vehicle, i.e. it is used for moving the motor vehicle, and further comprising an air compressor which supplies a compressed-air system of the vehicle, especially a hydrodynamic coupling which is interposed in the drive connection between the drive engine and the air compressor. The hydrodynamic coupling can be filled and discharged in order to thus switch the air compressor on and off, depending on the pressure state in the compressed-air system. The air compressor is arranged especially as a reciprocating-piston air compressor.
Compressed-air supply systems as related to the invention come with the advantage that as a result of the interposed hydrodynamic coupling an energetically advantageous cut-out of the air compressor is enabled on the one hand when a supply of the compressed-air system of the motor vehicle is not necessary as a result of sufficient pressure level in the same, such that the hydrodynamic coupling is “simply” discharged. On the other hand, vibration damping is effectively achieved by interposing the hydrodynamic coupling between the drive engine and the air compressor and it is reliably prevented that when using a reciprocating-piston air compressor a negative torque is transmitted back from the reciprocating-piston air compressor onto the drive engine or a transmission connected to the same, i.e. a torque which is generated by the compressor and which can occur in the region of the upper dead center of the reciprocating-piston air compressor.
In such a compressed-air supply system there is a plurality of states with different boundary conditions in which the various components of the compressed-air supply system need to cooperate in such a suitable manner that an energetically positive operation is ensured which protects the components. Different boundary conditions are for example the driving of the vehicle as compared with the standstill of the vehicle, the profile of the route which the vehicle is passing, such as driving up or down a hill, and different pressure states in the compressed-air supply system such as above a maximum permissible pressure, below the minimum permissible pressure and below a so-called release pressure at which the spring accumulator in the vehicle braking system is released and beneath which the vehicle is not allowed to drive. This release pressure is obtained by arranging the brakes as “fail safe”, which means that in the case of failure of the compressed-air system, the brake shoes are pressed by means of springs, so that braking occurs. From a certain air pressure the brake shoes can then be released actively, so that the vehicle is unable to drive away beneath this pressure.
In all known methods for regulating a compressed-air supply system of a motor vehicle in which the air compressor is driven by means of the drive engine it is common practice to set a fixedly predetermined minimum pressure value and a fixedly predetermined maximum pressure value for the pressure in the compressed-air system for any drive state of the motor vehicle. The pressure in the compressed-air system should always move between said two values, which means that when the pressure in the compressed air system has dropped to the minimum value, the air compressor is started, so that the compressed-air system is supplied. Once the pressure in the compressed-air system has reached the predetermined maximum pressure by said continued supply, the feeding is ended. As a result of this rigid predetermination of the two limit values it may occur that the vehicle is in the process of driving up a steep incline, which despite the full power output of the drive engine leads to an exceptionally low speed of the vehicle and at the same time the pressure in the compressed-air system falls beneath the predetermined minimum value as a result of the connected air consumers and the air compressor is thus started. Because the air compressor is also driven by the drive engine of the motor vehicle, the power intake of the air compressor required for this purpose is not available for driving the vehicle, and the vehicle continues to lose speed even further.
On the other hand, up till now the power intake of the air compressor has not been used in a purposeful energetic manner as a result of the rigidly predetermined limit values for switching the compressor on and off.
Reference is hereby made to the following documents in relation to the state of the art:    U.S. Pat. No. 6,036,449 A    U.S. Pat. No. 1,709,871 A    U.S. Pat. No. 4,459,085 A